Technical Field
The present invention relates to an ignition device that includes an ignition coil and a spark plug having a center electrode and a ground electrode that project into a combustion chamber of an internal combustion engine, in which a high voltage is applied to a gap that is a space between the center electrode and the ground electrode to generate a discharge spark.
Related Art
A spark plug for an internal combustion engine that includes a housing, a plug insulator, a center electrode, and a ground electrode is known. The outer periphery of the housing is provided with an attachment screw portion. The plug insulator is held within the housing. The center electrode is held within the plug insulator. The ground electrode forms a gap between itself and a tip portion of the center electrode. For example, the ground electrode is substantially L-shaped. The ground electrode is provided such as to oppose the tip portion of the center electrode in an axial direction of the spark plug.
As a result of a voltage being applied to the center electrode, an electron avalanche phenomenon occurs in the gap formed by the opposing portions of the center electrode and the ground electrode. The densities of free electrons and positive ions increase. As a result, breakdown occurs and a discharge spark is generated in the gap.
Within the combustion chamber of the internal combustion engine, airflow in a predetermined direction is generated during a compression step, for example, depending on the position of an intake port, the shape of the top surface of a piston, and the like. Here, when the flow of air-fuel mixture to the gap in the spark plug within the combustion chamber is obstructed, ignitability may decrease. Therefore, to prevent obstruction of the flow of air-fuel mixture by the ground electrode and improve ignitability, a technology is proposed in which the ground electrode is provided in a position in which the airflow of the air-fuel mixture is not obstructed (see JP-A-2005-299679).
On the other hand, when wear of the electrodes in the spark plug progresses as a result of increasing travel distance and the like and the gap widens, the voltage required to be applied to the gap to generate the discharge spark rises. As a result, the voltage applied to the gap may exceed the breakdown voltage of the plug insulator. Reliability of the spark plug may decrease.
As a measure against such issues, there is a technology in which the voltage applied to the center electrode is restricted to a predetermined voltage through use of a voltage regulator, such as a Zener diode or a varistor. Even when the applied voltage is restricted to the predetermined voltage, the densities of free electrons and positive ions increase in the gap between the electrodes as a result of the voltage being continuously applied to the center electrode. Breakdown occurs, and a discharge spark is generated in the gap. As a result, excessive increase of the voltage applied to the center electrode can be prevented. Decrease in the reliability of the spark plug can be suppressed.
When the voltage applied to the center electrode is restricted to a predetermined voltage, the spark plug can be protected. However, compared to when the restriction is not set, the amount of time from when the voltage is applied to the center electrode until the discharge spark is generated (discharge waiting period) increases.
Therefore, in a discharge waiting state, the likelihood increases that the positive ions produced in the gap will be carried away by the airflow of the air-fuel mixture during the compression step. As a result, a problem arises in that the amount of time from when voltage application to the center electrode starts until the discharge spark is generated varies depending on the state of airflow.